To estimate the maximum tolerated dose of the combination of temozolomide and BCNU in patients with central nervous system (CNS) or systemic tumors refractory to standard therapy; to characterize the dose-limiting toxicity and other toxicities of this combination when either temozolomide or BCNU is given first; to characterize the pharmacokinetics of temozolomide and any sequence effects on the pharmacokinetic parameters of temozolomide; and, to identify any preliminary evidence of anticancer activity in treated patients.